The invention relates generally to stents, and more particularly to coatings applied to stents to make them radiopaque and coatings to protect the stent and the radiopaque layer.
Stents are useful in the treatment of atherosclerotic stenoses in blood vessels and are generally tubular shaped devices which function to hold open a segment of a blood vessel, artery , heart valve or other body lumen. Stents are particularly suitable for use in supporting and holding open a coronary artery after an atherectomy or angioplasty procedure.
Generally, stents are made from a metal alloy, such as stainless steel, and have a hollow tubular shape with an outer wall surface resembling an open lattice configuration. In some prior art stents, the outer wall surface comprises intersecting wires or struts that are expanded beyond their elastic limit to plastically deform and hold open the body lumen in which they are implanted. Other stents are self-expanding and can be in the form of a coil wire that is biased open.
Stents made from stainless steel, for example, are radiolucent, due in part to the intersecting wires having a diameter of about 0.003 inch or less. Unless the metal or metal alloy used for making the stent has a high atomic weight and density, it is difficult to visualize in vivo during catheter introduction into the vessel, stent deployment, and post-operative diagnosis.
At least one prior art stent has an increased wire diameter, to approximately 0.004 inch, in order to make the stent more radiopaque. The disadvantages of a stent having thicker intersecting wires is a more rigid stent that tracks poorly through a tortuous vessel, is virtually inflexible when tracking on a curved section of vessel, it cannot be implanted easily in a curved section of a vessel, it may not deploy in a uniform cylindrical shape, and it has poor hemodynamics. The latter disadvantage, poor hemodyramics, can result in serious medical complications such as thrombosis.